Quantum computing is a class of computing in which inherently quantum mechanical phenomena, such as quantum state superposition and entanglement, are harnessed to perform certain computations far more quickly than any classical computer could ever be capable of. In a “topological” quantum computer, calculations are performed by manipulating quasiparticles—called “non-abelian anyons”—that occur in certain physical systems. Anyons have unique physical characteristics that distinguish them from both fermions and bosons. Non-abelian anyons also have unique properties with respect to abelian anyons. It is these unique properties that serve as a basis for topological quantum computing, in which information is encoded as a topological property of non-abelian anyons; specifically the braiding of their space-time worldlines. This has certain benefits over other models of quantum computation. One key benefit is stability, as the quantum braiding is unaffected by perturbations on a scale that could cause error-inducing quantum decoherence in other types of quantum computer.
A number of types of physical system have been considered as potential hosts of non-abelian anyons, such as “5/2 fractional quantum Hall” systems in condensed matter physics, and systems of topological insulators in contact with superconductors. Another example is semiconductor-superconductor (SE/SU) heterostructures such as SE/SU nanowires. With regard to these, a key advance in the field was the realization that non-abelian anyons, in the form of “Majorana zero modes” (MZMs), can be formed in regions where semiconductor (SE) is coupled to a superconductor (SU). Based on this phenomenon, a small network of SE/SU nanowires can be used to create a quantum bit, wherein each SE/SU nanowire comprises a length of semiconductor coated with a superconductor.
A quantum bit, or qubit, is an element upon which a measurement with two possible outcomes can be performed, but which at any given time (when not being measured) can in fact be in a quantum superposition of the two states corresponding to the different outcomes.
A “topological” qubit is a qubit implemented based on the above-mentioned technology of non-abelian anyons in the form of MZMs. A non-abelian anyon is a type of quasiparticle, meaning not a particle per se, but an excitation in an electron liquid that behaves at least partially like a particle. Particularly an anyon is a quasiparticle occurring in a two-dimensional system (two degrees of freedom in space). A Majorana zero mode is a particular bound state of such quasiparticles. Under certain conditions, these states can be formed close to the semiconductor/superconductor interface in an SE/SU nanowire network, in a manner that enables them to be manipulated as quantum bits for the purpose of quantum computing. Regions or “segments” of the nanowire network between the MZMs are said to be in the “topological” regime.
A Majorana-based qubit conventionally involves gating in order to exhibit such topological behaviour. That is, an electrical potential is applied to a segment of the semiconductor of one of the nanowires forming the qubit. The potential is applied via a gate terminal placed adjacent to the nanowire in the fabricated structure on the wafer. A magnetic field is also required to induce the topological regime. The magnetic field is applied from an electromagnet placed outside the wafer, typically within the refrigerating compartment as used to induce the superconductivity in the superconductor.
Conventionally, building Majorana-based topological quantum computing devices involves the formation of superconducting islands on the semiconductor. Some parts of the superconductor are topological (T) and some parts of which are non-topological (e.g., conventional S-wave (S)). The topological segment supports Majorana zero modes appearing at its opposite ends. The existing techniques for realizing MZMs require strong magnetic fields as well as electrostatic gating in order to drive the half-shell nanowires into the topological phase. The MZMs are induced by a coupling of the magnetic field to the spin component of the electrons. This requires a strong magnetic field.
In some fabrication techniques, the semiconductor of the nanowires may be formed in the plane of the wafer by a technique such as selective area growth (SAG). The superconducting material may then be deposited selectively over the semiconductor, or may be deposited as a uniform coating and regions subsequently etched away to form the islands.
Another method of fabricating a device comprising semiconductor-superconductor nanowires is disclosed in “Epitaxy of semiconductor-superconductor nanowires”, P. Krogstrup et al, Nature Materials, 12 Jan. 2015, pages 400-406. The semiconductor cores of the nanowires are grown vertically relative to the plane of the wafer, and then angle deposition is performed in order to deposit a coating of superconductor on facets of the semiconductor core. The nanowires are then “felled” by sonication and aligned in the horizontal plane by means of optical microscopy. Parts of the superconductor coatings are then etched away from the nanowires so as, in the resultant device, to leave each nanowire coated with just the superconducting islands mentioned above.